Optical sensors are widely used to determine the position of moving objects. One example is in an electricity meter wherein the speed of rotation of an eddy disk is related to the rate at which electricity is being consumed. See, for example, U.S. Pat. No. 4,827,123 assigned to the assignee of the present invention.
All optical sensors comprise a light emitter and a light detector that establish an optical path that includes a disk or other moving object. The moving object may either occlude the direct light from emitter to detector ("opto-interruptor"), or the moving object may, by virtue of its reflective and non-reflective areas, modulate the amount of reflected light passing from emitter to detector (reflective sensor).
Reference is now made to FIG. 1, wherein an optical sensor 20 comprises a light-emitting diode (LED) 22 converting an input electrical signal into light radiation, and a phototransistor 24 which is sensitive to light radiated by the LED 22. A rotating meter disk 26 is positioned so as to reflect incident light radiated by the LED 22 onto the phototransistor 24. The reflecting surface of the disk 26 has reflective areas 28a which reflect incident light therefrom, and non-reflective areas 28b which absorb light rather than reflect it. The conventional metallic surface of the disk is normally shiny enough to reflect light without any modification, whereas the non-reflective areas can be provided through a black applique applied to the disk. The output of the sensor is low when the phototransistor 24 is illuminated, and the output is high when the phototransistor 24 is not illuminated.
The LED 22 may be always kept on by providing a constant current at the input. However, the magnitude of the constant current is limited by the ability of the LED to operate with continuous excitation.
Alternatively, the LED 22 may be pulsed at intervals smaller than the expected light-to-dark period of the rotating disk 26. The intermittent mode of operation allows the LED to be operated at higher power to increase peak brightness.
Sensitivity of the optical sensor is defined by a signal-to-noise ratio R=(S+A)/(N+A), where:
S (signal) is the output phototransistor current resulting from light reflecting from a reflective area on the disk,
N (noise) is the output phototransistor current resulting from light reflecting from a non-reflective area on the disk, and
A (ambient) is the output phototransistor current resulting from background light.
In order to maximize R, the peak brightness should be as high as possible. Thus, the intermittent mode of operation allows a signal-to-noise ratio to be increased to improve sensitivity of optical sensors.
However, this mode of operation requires significant amount of power to be applied to the sensor in order to provide permanent LED pulsing with high peak brightness. Furthermore, permanent LED pulsing with high peak brightness results in reducing the mean time to failure (MTTF) of the LED, which is the average time before first failure of the LED. The MTTF is a measure of the reliability of the LED and the entire optical sensor.
In view of the above, it would be desirable to provide a method of and apparatus for controlling optical sensors to reduce power consumption without decreasing sensitivity. It would also be desirable to increase reliability of the optical sensor by increasing the mean time to failure of the LED.